踏足“未知的月球”，中国和世界要探索什么？The Far Side of the Moon: What China and the World Hope to Find

来源：纽约时报 2019-01-04 06:18

In a spaceflight first, China’s Chang’e-4 has landed where no spacecraft has touched down in one piece before: the far side of the moon.

经过一次前所未有的太空飞行，中国的嫦娥四号已经在此前航天器从未踏足的一片地方着陆：月球远面。

“This is a historic step in international scientific exploration of the moon, opening up the ‘Luna Incognita’ of the lunar far side to surface exploration for the first time,” said James Head, a planetary scientist at Brown University.

If successful, the mission could answer fundamental questions about Earth’s only natural satellite. There are still mysteries, for example, about the moon’s formation and early evolution, which, in turn, hold clues to the history of the entire solar system.

Additionally, the mission will conduct the first radio astronomy experiments from the moon’s far side and the first investigations to see whether plants can grow on the moon — a crucial step toward long-term human missions beyond Earth.

If you were to watch the moon over the course of a month, the “man in the moon” would never truly disappear. Despite the moon’s phases, he’s always there, keeping a watchful eye over you from his heavenly perch. That’s because the moon is tidally locked with Earth. It rotates exactly once every time it circles our planet, thus keeping the same hemisphere pointing toward Earth at all times.

Astronomers refer to the side we always see from Earth as the “near side” and the side we can never see as the “far side.”

天文学家将我们总是从地球上看到的那一面称为“近面”，而我们永远看不到的那一面称为“远面”。

It’s not really the “dark side”

它并非真的是“暗面”

While the far side can never be seen from Earth, it is still illuminated by the sun and has the same phases as the near side. There is no permanently “dark side” of the moon, although it has been described this way in popular culture to refer to the moon’s unknown side.

When the Soviet spacecraft Luna 3 sent back the first images of the far side of the moon in 1959, it revealed a world that looked vastly different from the one we see.

1959年，当苏联的月亮3号(Luna 3)太空船发回第一批月球远面的照片时，它揭示了一个和我们所看到的大不相同的世界。

The face of the “man in the moon” on the near side is so noticeable because it’s composed of dark areas, which stand out against the light lunar soil. Those dark areas formed when ancient asteroids struck the moon’s surface, unleashing lava that darkened the facade and smoothed it, erasing the records from previous impacts.

But when ancient asteroids struck the far side of the moon, there were no floods of lava. The impacts simply left a surface pockmarked with craters. That makes the far side much lighter, much older and much more heavily cratered than the near side.

但当远古小行星撞击月球远面时，却没有熔岩洪流。其影响只是给表面留下了陨石坑的痕迹。这使得远面比近面更亮、更苍老、陨石坑痕迹更重。

Some astronomers suspect the dichotomy arises because the crust on the near side is much thinner than that of the far side. That would make it easier for magma to emerge on the near side, explained Briony Horgan, a planetary scientist at Purdue University. But why the crust’s thickness would vary so drastically from one hemisphere to the next remains a mystery. And Dr. Horgan is hopeful that Chang’e-4 will provide hints to the answer.

The early history of the solar system was violent. That’s when large objects — asteroids or comets — pounded the rocky planets and left craters, some that are more than 600 miles across.

太阳系的早期十分动荡。小行星或彗星等大的物体撞击了岩质行星，留下了陨石坑，有的直径超过600英里。

But most evidence of this tumultuous past has disappeared, erased by time. On Earth and other rocky worlds, volcanoes have washed away these craters over billions of years.

但这段混乱历史的大部分证据都消失了，被时间抹去。在地球和其它岩质星球上，这些陨石坑被火山在数十亿年间里冲刷掉了。

The far side of the moon has retained a pristine record of its youth, particularly the number of times that ancient objects pummeled its surface.

月球远面则保留了其年轻的原始记录，特别是古老物体撞击其表面的次数。

“The history of the very early solar system is locked up in the rocks of the far side,” Dr. Horgan said.

“太阳系初期的历史就锁在远面的岩石中”，霍根说。

Land on the far side, see the universe anew

着陆远面，重观宇宙

Earth is loud. Any radio antenna will reveal a symphony of strange noises that emanate from cellphones, TV stations, power lines, electrical fences, distant lightning, GPS satellites, cars, Wi-Fi and, well, radios, to name a few. But the moon shields the far side from much of that chatter, thus creating the ideal location to study the radio universe.

That’s why astronomers have long dreamed of building a radio telescope on the far side of the moon.

正因为如此，天文学家们长期以来都梦想着在月球远面建造一个无线电天文望远镜。

“We use radio wavelengths to probe everything from black holes in the local neighborhood to distant galaxies, so a radio observatory on the far side could be a huge boon to astronomy,” Dr. Horgan said. “The Chang’e-4 mission will be the first mission to test out this theory and to see just how much better the lunar far side is for radio observations than our observatories back on Earth.”

A future lunar observatory — if it works as intended — could improve the study of phenomena like the primordial clouds of hydrogen gas that coalesced into the universe’s first stars.

如能按预期运转，那么未来的月球天文台将促进凝结到第一代恒星当中的原始氢气云这类现象的研究。

“We would be able to listen to a distant echo of the Big Bang and see the universe in a state before the first-ever stars formed,” said Heino Falcke, a radio astronomer at Radboud University Nijmegen in the Netherlands.

Such a signal would allow astronomers to probe the early development of the universe. Dr. Falcke pointed to the famous cosmic heat map of the universe as it appeared only 370,000 years after the Big Bang, also known as the cosmic microwave background. “That was a baby picture,” he said, but from the far side of the moon it might be possible to, “see a movie of the evolution of the young universe from toddler to puberty.”

Chang’e-4 is also carrying an onboard experiment to test how well plants — specifically potatoes and small flowering Arabidopsis plants — grow on the moon, which could contribute to China’s human spaceflight goals. It is the first mini-greenhouse to land on another world in the solar system.

Anna-Lisa Paul, a horticultural scientist at the University of Florida in Gainesville, said that the tiny experiment “is a step forward in preparing people to return to the moon for longer than a brief visit.”